In the MOS (Metal-Oxide Semiconductor) transistor, an extremely thin gate oxide film or layer is used to improve electric characteristics of the element. At present, a gate oxide film having the thickness of 60 through 70 angstroms is used in a semiconductor device relying on 0.25 micrometer (micron) rule, and a gate oxide film having the thickness of 30 through 40 angstroms is used in a semiconductor device relying on 0.18 micrometer rule. Further, according to the increase in the density of integration of a semiconductor device, a gate oxide film having the thickness of 20 through 30 angstroms is required in a semiconductor device relying on 0.13 micrometer rule.
Since the gate oxide film influences reliability and electrical characteristics of a semiconductor device, it is required that time-zero breakdown voltage is high, that time-dependent degradation is small and that there is no defect at the interface between a semiconductor substrate and the gate oxide film. These characteristics seem to be most superior in case of a thermal oxide film formed by using dry oxidation, that is, oxidation using O2. According to the dry oxidation, Si substrate is heated in O2 ambient or atmosphere, and it is possible to easily form SiO2 film having superior insulating characteristic.
In order to make thin SiO2 film, three methods are conceivable. That is, a method by decreasing oxidation temperature, a method by decreasing oxidation time and a method by decreasing oxidation pressure. Conventionally, among these methods, two methods, i.e., the method by adjusting, i.e., decreasing, oxidation temperature (temperature of heating Si substrate in oxidation ambient) and the method by adjusting, i.e., decreasing, oxidation time (time of heating Si substrate in oxidation ambient) have been used for adjusting the thickness of the SiO2 film formed.
On the other hand, required tolerance of the thickness of the gate oxide film of a MOS transistor becomes strict as the thickness of the gate oxide film becomes thin. For example, when the nominal thickness of a gate oxide film is 60 angstroms, the required tolerance of the thickness of the film is +/-3 angstroms. When the nominal thickness of a gate oxide film is 40 angstroms, the required tolerance of the thickness of the film is +/-2 angstroms. When the nominal thickness of a gate oxide film is 20 angstroms, the required tolerance of the thickness of the film is +/-1 angstrom. In this case, the required tolerance includes dispersion limit of the thickness of the oxide film over a plane of a Si substrate and dispersion limit of the thickness of the oxide films between substrates.
In order to easily fabricate a gate oxide film having high reliability, which is free from inferiority of time-zero breakdown voltage and free from time-dependent degradation and which has a desired film thickness within a required tolerance, the inventor of the present invention newly considered a method for controlling the thickness of a gate oxide film. When oxidation temperature was reduced to realize an oxide film having thin film thickness, it became apparent that there were problems of low time-zero breakdown voltage of the gate oxide film and of increase in the time-dependent degradation. Therefore, when thin SiO2 film is to be formed, it is not preferable to control the film thickness of the SiO2 film by adjusting the oxidation temperature.
The inventor of this application then fabricated a SiO2 film on a Si substrate by thermally oxidizing the Si substrate in oxygen ambient at an oxidation temperature of 1000 degrees Celsius, that is, at a constant oxidation temperature. FIG. 1 is a graph illustrating relation between the formed film thickness of the SiO2 film and oxidation time, i.e., time period heated at 1000 degrees Celsius. In FIG. 1, the relation is shown at each of oxidation pressures, i.e., pressures of oxygen ambient, of 760, 100, 50, 20, and 10 Torr. As shown in FIG. 1, when the oxidation time is changed, the film thickness of the oxide film, i.e., SiO2 film, formed by thermal oxidation depends on the oxidation time. As the oxidation time becomes shorter, the film thickness of the SiO2 becomes thinner. From FIG. 1, it is also found that, as the oxidation pressure becomes lower, the thinner SiO2 film can be formed, when the oxidation time is the same.
As apparent from FIG. 1, when the oxide film having the thickness equal to or thinner than 30 angstroms is to be formed, it is necessary to perform thermal oxidation at a condition of reduced pressure or a decompressed condition, that is, in an oxygen ambient having a pressure less than 760 Torr. For example, when the gate oxide film having the thickness of 20 angstroms required in the 0.13 micrometer rule is to be fabricated, the oxidation pressure is reduced, for example, to 50 Torr, and Si substrate is heated for 20 seconds at 1000 degrees Celsius to form the oxide film. If the oxidation pressure is further reduced, the oxidation time can be elongated.
On the other hand, as the oxidation pressure becomes low, heat conduction through gas molecules is suppressed. Therefore, temperature distribution throughout the surface of the Si substrate becomes non-uniform, so that dispersion of thickness of the formed oxide film within the substrate becomes large. When the oxidation pressure is raised as high as possible to avoid such non-uniformity when the Si substrate is thermally oxidized, it is necessary to shorten oxidation time. However, as apparent from FIG. 1 showing curves illustrating relation between the formed thickness of the SiO2 film and the oxidation time, forming rate of the SiO2 film after some length of time has elapsed, that is, in a time region shown by the curves of FIG. 1, is relatively slow. On the other hand, at the initial stage of the oxide film formation, that is, in a time region not shown by the curves of FIG. 1, forming rate of the SiO2 film is large and a large quantity of SiO2 film is formed in a relatively short time. This is apparent from the fact that the formed thickness of the oxide film is zero when the oxidation time is zero in FIG. 1. Therefore, when the formed thickness of the oxide film is controlled by adjusting the oxidation time and thin gate oxide film is required, it is necessary to adjust the oxidation time to a very short time and to control the oxidation time very precisely.
However, when the oxidation should be performed for a very short time, it is difficult to precisely control the thickness of the oxide film. For example, if a resistance heating furnace is used, the oxide film grows even while taking the Si substrate into and out of the furnace. Therefore, when the oxide film having the thickness equal to or lower than 50 angstroms is formed by this method, it is difficult to form the oxide film precisely in a predetermined film thickness. Also, even if a rapid heating furnace, such as a lamp heating furnace, which can raise and drop temperature in a short time, is used, it is difficult to control the very short oxidation time precisely. Therefore, even when the rapid heating furnace is used, it is difficult to precisely fabricate the oxide film in a predetermined thickness, if the thickness of the oxide film should be, for example, equal to or thinner than 30 angstroms, and if the thickness of the oxide film is controlled by adjusting the oxidation time.